The present invention relates to method and apparatus for harmonic free generation in multiple mixing frequency conversions.
One of the major functions of the front-end of a receiver is to provide a frequency conversion from the input RF frequency to a pre-defined IF frequency or baseband. The purpose of the frequency conversion is to perform the demodulation at a fixed center frequency.
There are many different conversions suitable for this purpose. The selection of the specific type of conversion depends on the specific application, performance specifications and cost sensitivity. RF specifications may allow or prevent utilization of a specific type of conversion. For example, sensitivity to local oscillator (xe2x80x9cLOxe2x80x9d) leakage into the RF signal may prevent using a direct conversion scheme, where the RF signal is converted directly to baseband, no matter how efficient the conversion is.
The CATV environment consists of multiple wideband digital and analog carriers, typically covering a frequency range of 54-860 MHz. A receiver for digital modulation is required to demodulate the incoming signal, in the presence of many other signals, some of which are higher in power, and still meet a desired performance. Transmission of digital signals over a cable typically utilizes a very efficient digital modulation, such as 256QAM, where a very high signal-to-noise ratio is required. As a result, the receiver front-end must have a very high rejection of interference from other existing signals, such as images and adjacent signals. For example, if a 256QAM signal requires a signal-to-noise ratio of about 30 dB and is demodulated in the presence of an image which is +10 dBc, the image rejection should be at least 50 dB, for the resulting interference to be 10 dB below the signals"" noise level. In such cases, a single conversion may be used to a fixed IF, but will have to use narrowband tracking filters and image rejection mixers to achieve the overall 50 dB image rejection over the full frequency band. Another option is to use a double conversion, where the signal is upconverted to a fixed frequency, filtered with a fixed filter and downconverted to the required IF. The double conversion provides an excellent image rejection but is achieved at an additional cost.
The use of RF CMOS technology for the tuner for digital receivers in a CATV environment offers the ability to integrate all tuner functions into one silicon solution, with a clear cost advantage. On the other hand, other implementation drawbacks arise when such a solution is considered. The isolation and filtering capabilities of RF CMOS are inherently poor. If the use of external components, such as narrowband filters, is to be avoided, then the design of the frequency conversion must be robust enough to minimize relevant interference sources which may reduce the filtering and isolation specifications. A conventional implementation of such an integrated conversion is depicted in FIG. 1.
A double conversion scheme, to a high 2nd IF frequency (a few hundred MHz), with an additional complex conversion to baseband, is a candidate for a full integration of the tuner into silicon; this eliminates external components, such as the narrowband fixed filter in the 1st IF. If the 2nd IF frequency is high enough, the image filtering in the 1st IF becomes feasible. However, double conversion presents many other noise sources that contribute to overall noise and spurious signal levels. Some of the basic sources of interference in such a double conversion are:
The present invention provides a frequency selection method (i.e. selection of 1st, 2nd IF frequencies for a given signal bandwidth BW and input frequency range RFmin to RFmin) that inherently isolates and filters major interference sources. The frequency selection method specifically addresses the conversion of out-of-band signals due to local oscillator harmonics, in a double conversion scheme. The method provides a frequency range free of harmonics (i.e. a xe2x80x9cFree harmonic rangexe2x80x9d, e.g. a 3rd order free harmonic range) and with this, relaxes the filtering requirements for both the input and the 1st IF section (e.g. eliminates the need for external narrowband filter in the 1st IF section). The frequency selection of the present invention method therefore enables integration of the full tuner into one piece of silicon.